I was wondering about that too. It doesn't really say in the manual. However, in the manual for the GTP 1 it mentions 10mm. I downloaded that manual and don't remember where. What I was wondering is when I adjust for the ride height using the adjusters located on the lower arms(front and rear), it compresses the front shocks so much they seem useless with only a couple millimetres of travel. I have the rtr GTP-2 and have not changed the weight of the shock oil.
PS....How do you like your GTP-2?

adjust the ride height by using the spring adjusters. Do not use the downstop screws in the arms. Start with about 10mm in front and 12-14mm in rear. The ride height you can attain will also depend on the tires you are using different tires and brands can have a different diameter.